 LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
 subroutine stbt03 ( character UPLO, character TRANS, character DIAG, integer N, integer KD, integer NRHS, real, dimension( ldab, * ) AB, integer LDAB, real SCALE, real, dimension( * ) CNORM, real TSCAL, real, dimension( ldx, * ) X, integer LDX, real, dimension( ldb, * ) B, integer LDB, real, dimension( * ) WORK, real RESID )

STBT03

Purpose:
``` STBT03 computes the residual for the solution to a scaled triangular
system of equations  A*x = s*b  or  A'*x = s*b  when A is a
triangular band matrix. Here A' is the transpose of A, s is a scalar,
and x and b are N by NRHS matrices.  The test ratio is the maximum
over the number of right hand sides of
norm(s*b - op(A)*x) / ( norm(op(A)) * norm(x) * EPS ),
where op(A) denotes A or A' and EPS is the machine epsilon.```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 Specifies whether the matrix A is upper or lower triangular. = 'U': Upper triangular = 'L': Lower triangular``` [in] TRANS ``` TRANS is CHARACTER*1 Specifies the operation applied to A. = 'N': A *x = b (No transpose) = 'T': A'*x = b (Transpose) = 'C': A'*x = b (Conjugate transpose = Transpose)``` [in] DIAG ``` DIAG is CHARACTER*1 Specifies whether or not the matrix A is unit triangular. = 'N': Non-unit triangular = 'U': Unit triangular``` [in] N ``` N is INTEGER The order of the matrix A. N >= 0.``` [in] KD ``` KD is INTEGER The number of superdiagonals or subdiagonals of the triangular band matrix A. KD >= 0.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrices X and B. NRHS >= 0.``` [in] AB ``` AB is REAL array, dimension (LDAB,N) The upper or lower triangular band matrix A, stored in the first kd+1 rows of the array. The j-th column of A is stored in the j-th column of the array AB as follows: if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j; if UPLO = 'L', AB(1+i-j,j) = A(i,j) for j<=i<=min(n,j+kd).``` [in] LDAB ``` LDAB is INTEGER The leading dimension of the array AB. LDAB >= KD+1.``` [in] SCALE ``` SCALE is REAL The scaling factor s used in solving the triangular system.``` [in] CNORM ``` CNORM is REAL array, dimension (N) The 1-norms of the columns of A, not counting the diagonal.``` [in] TSCAL ``` TSCAL is REAL The scaling factor used in computing the 1-norms in CNORM. CNORM actually contains the column norms of TSCAL*A.``` [in] X ``` X is REAL array, dimension (LDX,NRHS) The computed solution vectors for the system of linear equations.``` [in] LDX ``` LDX is INTEGER The leading dimension of the array X. LDX >= max(1,N).``` [in] B ``` B is REAL array, dimension (LDB,NRHS) The right hand side vectors for the system of linear equations.``` [in] LDB ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N).``` [out] WORK ` WORK is REAL array, dimension (N)` [out] RESID ``` RESID is REAL The maximum over the number of right hand sides of norm(op(A)*x - s*b) / ( norm(op(A)) * norm(x) * EPS ).```
Date
November 2011

Definition at line 177 of file stbt03.f.

177 *
178 * -- LAPACK test routine (version 3.4.0) --
179 * -- LAPACK is a software package provided by Univ. of Tennessee, --
180 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
181 * November 2011
182 *
183 * .. Scalar Arguments ..
184  CHARACTER diag, trans, uplo
185  INTEGER kd, ldab, ldb, ldx, n, nrhs
186  REAL resid, scale, tscal
187 * ..
188 * .. Array Arguments ..
189  REAL ab( ldab, * ), b( ldb, * ), cnorm( * ),
190  \$ work( * ), x( ldx, * )
191 * ..
192 *
193 * =====================================================================
194 *
195 * .. Parameters ..
196  REAL one, zero
197  parameter ( one = 1.0e+0, zero = 0.0e+0 )
198 * ..
199 * .. Local Scalars ..
200  INTEGER ix, j
201  REAL bignum, eps, err, smlnum, tnorm, xnorm, xscal
202 * ..
203 * .. External Functions ..
204  LOGICAL lsame
205  INTEGER isamax
206  REAL slamch
207  EXTERNAL lsame, isamax, slamch
208 * ..
209 * .. External Subroutines ..
210  EXTERNAL saxpy, scopy, slabad, sscal, stbmv
211 * ..
212 * .. Intrinsic Functions ..
213  INTRINSIC abs, max, real
214 * ..
215 * .. Executable Statements ..
216 *
217 * Quick exit if N = 0
218 *
219  IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
220  resid = zero
221  RETURN
222  END IF
223  eps = slamch( 'Epsilon' )
224  smlnum = slamch( 'Safe minimum' )
225  bignum = one / smlnum
226  CALL slabad( smlnum, bignum )
227 *
228 * Compute the norm of the triangular matrix A using the column
229 * norms already computed by SLATBS.
230 *
231  tnorm = zero
232  IF( lsame( diag, 'N' ) ) THEN
233  IF( lsame( uplo, 'U' ) ) THEN
234  DO 10 j = 1, n
235  tnorm = max( tnorm, tscal*abs( ab( kd+1, j ) )+
236  \$ cnorm( j ) )
237  10 CONTINUE
238  ELSE
239  DO 20 j = 1, n
240  tnorm = max( tnorm, tscal*abs( ab( 1, j ) )+cnorm( j ) )
241  20 CONTINUE
242  END IF
243  ELSE
244  DO 30 j = 1, n
245  tnorm = max( tnorm, tscal+cnorm( j ) )
246  30 CONTINUE
247  END IF
248 *
249 * Compute the maximum over the number of right hand sides of
250 * norm(op(A)*x - s*b) / ( norm(op(A)) * norm(x) * EPS ).
251 *
252  resid = zero
253  DO 40 j = 1, nrhs
254  CALL scopy( n, x( 1, j ), 1, work, 1 )
255  ix = isamax( n, work, 1 )
256  xnorm = max( one, abs( x( ix, j ) ) )
257  xscal = ( one / xnorm ) / REAL( kd+1 )
258  CALL sscal( n, xscal, work, 1 )
259  CALL stbmv( uplo, trans, diag, n, kd, ab, ldab, work, 1 )
260  CALL saxpy( n, -scale*xscal, b( 1, j ), 1, work, 1 )
261  ix = isamax( n, work, 1 )
262  err = tscal*abs( work( ix ) )
263  ix = isamax( n, x( 1, j ), 1 )
264  xnorm = abs( x( ix, j ) )
265  IF( err*smlnum.LE.xnorm ) THEN
266  IF( xnorm.GT.zero )
267  \$ err = err / xnorm
268  ELSE
269  IF( err.GT.zero )
270  \$ err = one / eps
271  END IF
272  IF( err*smlnum.LE.tnorm ) THEN
273  IF( tnorm.GT.zero )
274  \$ err = err / tnorm
275  ELSE
276  IF( err.GT.zero )
277  \$ err = one / eps
278  END IF
279  resid = max( resid, err )
280  40 CONTINUE
281 *
282  RETURN
283 *
284 * End of STBT03
285 *
integer function isamax(N, SX, INCX)
ISAMAX
Definition: isamax.f:53
subroutine stbmv(UPLO, TRANS, DIAG, N, K, A, LDA, X, INCX)
STBMV
Definition: stbmv.f:188
subroutine saxpy(N, SA, SX, INCX, SY, INCY)
SAXPY
Definition: saxpy.f:54
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:55
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:69
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:55
subroutine scopy(N, SX, INCX, SY, INCY)
SCOPY
Definition: scopy.f:53

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